1. Field of the Invention
The present invention relates to systems and methods for guidance and navigation of spacecraft and in particular to a system and method for refining the attitude and angular velocity estimates of a spacecraft.
2. Description of the Related Art
Spacecraft typically have one or more payloads that are directed to transmit or receive energy from ground stations. For example, communication satellites include one or more uplink antennae for receiving information from an uplink center, and one or more downlink antennae for transmitting information to a terrestrial receiver. The uplink and downlink antennae are typically disposed on the satellite body (or spacecraft bus) and are directed toward a terrestrial location where an uplink/downlink antenna is transmitting/receiving the information.
In order to perform their intended functions, the attitude of such satellites must be accurately determined and controlled. Advanced satellite systems typically require attitude determination and control under more demanding circumstances and with greater accuracy. Such advanced satellite systems may be required to perform attitude determination under adverse conditions (e.g. while the satellite is experiencing rapid attitude changes and/or in high radiation environments), and in response to a loss of an attitude estimate, may be asked to perform an attitude determination autonomously. While methods exist to recover attitude estimates, the accuracy of the estimate obtained may not be sufficient to meet system requirements. Furthermore, the attitude estimate obtained must be updated as the satellite moves; this requires the systems to also estimate satellite angular velocity, with respect to inertial space. As with the attitude determination, the difficulty is not in estimating an angular velocity, but in estimating it accurately enough to satisfy requirements. In the future, satellites will be required to perform these functions at more stringent performance requirement levels (measured by such metrics as time to acquire attitude and angular velocity estimates and the accuracies of the estimates) than are generally required today.
In the past, to meet even the near term performance requirements for attitude determination under adverse conditions, satellites have required additional spacecraft sensors (such as spinning earth sensors), and intense ground processing of telemetered data.
What is needed is a system and method for refining inaccurate satellite attitude and angular velocity estimates under demanding circumstances without the use of sensors beyond gyro and star trackers (which are used for normal mode attitude estimation and control) and without ground support. The present invention satisfies that need.